This invention relates to an optical imaging Fourier spectrometer and method of operating it.
Fourier transform spectroscopy (FTS) is a long-established method for obtaining the absorption or emission spectra of substances. In this method, light from a specimen is passed to a detector via an optical interferometer, often of the Michelson type. The light intensity at the output is measured while the length of the light path in one arm of the interferometer is varied. The spectrum is then obtained by computation as the reverse Fourier transform of the intensity profile as described by R. J. Bell (1972) in the textbook Introductory Fourier Transform Spectroscopy published by Academic Press, New York. Recently, a novel form of FTS has been described by Yuval Garini and others in U.S. Pat. No. 5,539,517; also in Chapter 4, pp 87-124, of Fluorescence Imaging Spectroscopy and Microscopy edited by Xue Wang and Brian Herman, Chemical Analysis Series, Volume 137 published by John Wiley and Sons Inc.
In the latter, a field (which may be a microscope image) is imaged in a camera, using light that has passed through a Sagnac interferometer. During the recording, the interferometer is adjusted in such a way that a series of interference fringes passes in a precise and controlled fashion across the field of the camera and is recorded. This series corresponds, in the case of an initial input of white light, to Newton""s series. With this apparatus, it is possible to obtain by means of a computer, an independent Fourier transform spectrum for each pixel of the set of images obtained. This information is of potential value in many applications, but it has notably been applied to the detection of multiple coloured fluorescent dyes bound to biological specimens in a light microscope.
The prior art works well, but suffers from two defects. Firstly, the position of the fringes in the interferometer is sensitive to very small displacements of the optical elements, corresponding to shifts of the order of a wavelength of light. This makes the Sagnac system difficult to align and to keep in alignment during transport. Secondly, the transmission efficiency of the Sagnac system is maximally only fifty percent, since half of the light put into it passes back towards the source: this is an intrinsic property of this type of interferometer. The present invention offers the possibility of improvement in both these respects.
Minami in Mikrochim. Acta [Wien] 1987, III, 309-324 has discussed the use of birefringent optical techniques rather than an interferometer in analysis of an aperture source. Here, instead of a physical difference in optical path length it is the retardation due to birefringence that is varied. However, he lacks the capacity to record any image of a field.
Although Minami uses a CCD camera to image the fringe system in his spectrometer, he is using it to record one-dimensional information; there is no image of a field, whether coincident with that of the fringe system or otherwise. The resulting interferogram allows analysis of the spectral quality of light gathered by a condenser lens onto a circular aperture but does not allow spectral information to be obtained simultaneously from multiple regions of a field so that, for example, comparisons can be made of one region with another.
According to the invention, imaging FTS is carried out in an apparatus wherein at least one birefringent device is placed in an optical light path between polarizing devices, with image-generating andxe2x80x94recording apparatus respectively before and after the polarizing devices, the image-generating apparatus generating a real image in the same plane or planes as fringes generated by the birefringent device(s), and means for causing change in the optical path difference suffered by light in the birefringent device(s).
In this way there can be a systematic change due to variation in optical retardation throughout the recorded images in a controlled fashion, so that the Fourier spectrum of each element xe2x80x9cpixelxe2x80x9d of an image-recording camera can be obtained by known means of computation, from the variation in intensity of the elements across the set of recorded images.
In other words, FTS is performed using a birefringent device to vary the optical path difference systematically for all regions of an image simultaneously, so that a separate interferogram is collected for each image region by combining intensity values in multiple images.
The invention includes an embodiment in which the Wollaston prism, quartz wedge or other birefringent device or devices are fabricated such that the optical slow direction is at an angle to the mechanical long axis.
There may be complementary optical paths with a common image-generating apparatus and at least one common image-recording apparatus; in this case it is advantageous to use birefringent devices with a slow direction at 45xc2x0 to the mechanical long axis in the respective paths. This allows the fringe system to be produced and caused to move without the necessity for quarter-wave plates or any other additional optical elements. A polarizing beamsplitter is preferred as the polarizer on the recording side of the birefringent device.
The invention also includes a method of generating a Fourier transform in imaging spectrometry which includes passing light from a field (which may be a real image) through a polarizer to form a real image in the same plane as the fringes of a birefringent device through which the light passes, and then through a second polarizer to an image-receiver, controlledly varying the optical path difference in the birefringent device, recording the variation of intensity in the receiver in relation to the variation of optical path difference, and computing the Fourier spectrum of the elements of the recorded images.
The advantage of the general design and method over the interferometer-based prior art is its lack of sensitivity to mechanical and thermal stress and to errors in the placement of its mechanical parts while allowing the analysis of information from the whole of a field. For example, in those embodiments which have fringes crossing the image, by appropriate choice of the wedge angles and/or dimensions of a birefringent element, the fringe separation can be made as large as desired. Compared with the interferometer, the direction and separation of the fringes are almost completely insensitive to environmental influences and to misplacements of the optics. Also, the fringes can be translated without any need for high mechanical precision in the slide or screw mechanism. In the preferred embodiment, a marker, e.g. in the form of a line, is fixed on or near the moveable birefringent optical element and moves with it in such a way as to indicate the position of the fringe system either via the image in a camera or through the action of some other type of sensor, switch or mechanical device. This marker serves as a guide to the identification of the so-called xe2x80x9czero order fringexe2x80x9d or centre of the fringe pattern and so facilitates computation of spectra from a series of recorded images. Since the position of the fringes in the prior art of Garini et al is not determined by the position of any single optical element such a marker cannot easily be incorporated into the prior art.
It is also possible, in certain embodiments, for the optical efficiency to approach 100 per cent.